CPClimate of the PastCPClim. Past1814-9332Copernicus PublicationsGöttingen, Germany10.5194/cp-15-41-2019Comparing the spatial patterns of climate change in the 9th and
5th millennia BP from TRACE-21 model simulationsComparing the spatial patterns of climate changeNingLiangLiuJianjliu@njnu.edu.cnBradleyRaymond S.YanMi1Key Laboratory of Virtual Geographic Environment, Ministry of
Education, State Key Laboratory of Geographical Environment Evolution,
Jiangsu Provincial Cultivation Base, School of Geographical Science, Nanjing
Normal University, Nanjing, China2Jiangsu Center for Collaborative Innovation in Geographical
Information Resource Development and Application, Nanjing, China3Climate System Research Center, Department of Geosciences, University
of Massachusetts, Amherst, MA, USAJian Liu (jliu@njnu.edu.cn)10January2019151415230September201811October201813December2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://www.clim-past.net/15/41/2019/cp-15-41-2019.htmlThe full text article is available as a PDF file from https://www.clim-past.net/15/41/2019/cp-15-41-2019.pdf

The spatial patterns of global temperature and precipitation changes, as well
as corresponding large-scale circulation patterns during the latter part of
the 9th and 5th millennia BP (4800–4500 versus 4500–4000 BP and
9200–8800 versus 8800–8000 BP) are compared through a group of transient
simulations using the Community Climate System Model version 3 (CCSM3). Both
periods are characterized by significant sea surface temperature (SST)
decreases over the North Atlantic, south of Iceland. Temperatures were also
colder across the Northern Hemisphere but warmer in the Southern Hemisphere.
Significant precipitation decreases are seen over most of the Northern
Hemisphere, especially over Eurasia and the Asian monsoon regions, indicating
a weaker summer monsoon. Large precipitation anomalies over northern South
America and adjacent ocean regions are related to a southward displacement of
the Intertropical Convergence Zone (ITCZ) in that region. Climate changes in
the late 9th millennium BP (the “8.2 ka event”) are widely considered to have been caused by a large freshwater
discharge into the northern Atlantic, which is confirmed in a meltwater
forcing sensitivity experiment, but this was not the cause of changes
occurring between the early and latter halves of the 5th millennium BP.
Model simulations suggest that a combination of factors, led by long-term
changes in insolation, drove a steady decline in SSTs across the North
Atlantic and a reduction in the North Atlantic Meridional Overturning
Circulation (AMOC), over the past 4500 years, with associated teleconnections
across the globe, leading to drought in some areas. Multi-century-scale
fluctuations in SSTs and AMOC strength were superimposed on this decline.
This helps explain the onset of neoglaciation around 5000–4500 BP, followed
by a series of neoglacial advances and retreats during recent millennia. The
“4.2 ka BP Event” appears to have been one of several late Holocene
multi-century fluctuations that were embedded in the long-term, low-frequency
change in climate that occurred after ∼4.8 ka. Whether these
multi-century fluctuations were a response to internal centennial-scale
ocean–atmosphere variability or external forcing (such as explosive volcanic
eruptions and associated feedbacks) or a combination of such conditions is
not known and requires further study.

Introduction

It is well documented that the first-order driver of Holocene climate change
was orbital forcing, with an overall decline in summer insolation in summer
months, particularly at high latitudes. This led to a drop in temperatures at
high latitudes and less rainfall throughout the monsoon regions of the
Northern Hemisphere, as seen in many paleoclimatic records (Burns, 2011;
Solomina et al., 2015). Shorter-term rainfall fluctuations superimposed on
this long-term change in hydrological conditions are clearly seen in many
speleothem and lacustrine sediment records (e.g., Wang et al., 2005; Kathayat
et al., 2017). Abrupt hydrological changes around 4.2 ka have been
documented for various regions of the world; it has been suggested that the
major global monsoon and ocean–atmosphere circulation systems were deflected
or weakened synchronously at this time, causing major century-scale
precipitation disruptions (severe megadroughts) over different regions
(Weiss, 2017). Other studies (Wang et al., 2005; Tan et al., 2018a) have also
noted weakening of the Asian summer monsoon at around this time, resulting in
drought over the northern part of eastern China and flooding over the
southern part.

In recent years, a more comprehensive picture of the “4.2 ka BP Event”
has been derived from analysis of new high-resolution proxy data from
different regions, and the event has become the focus of symposia and
research conferences (e.g., Weiss, 2015). This event is of particular interest
as it is associated with societal collapse and regional abandonment in many
different regions. For example, the collapse and abandonment of Akkadian
imperial settlements in the Khabur Plains, and other communities in dry
farming domains across the Aegean and west Asia, was in response to the
abrupt nature with which the megadrought began (with its onset in less than
5 years), its magnitude (a precipitation reduction of 30 %–50 %),
and its long duration (200–300 years) (Weiss, 2017).

Although a drought episode around 4.2 ka has been found in many proxy
reconstructions, the mechanisms that brought this about are still unclear,
though different hypotheses have been proposed. For example, Staubwasser and
Weiss (2006) suggested that the abrupt climate change event at 4.2 ka, as
well as other widespread droughts around 8.2 and 5.2 ka over the eastern
Mediterranean, west Asia, and the Indian subcontinent, was caused by a change
in subtropical upper-level flow over the eastern Mediterranean and Asia. Some
studies have suggested that these large-scale circulation anomalies may
reflect persistent modes of internal climate variability, though there is a
wide range of other explanations. For example, Booth et al. (2005) indicated
that the widespread midlatitude and subtropical drought around 4.2 ka was
linked to a La Niña-like sea surface temperature (SST) pattern, possibly
associated with amplification of this spatial mode by variations in solar
irradiance or volcanism. On the other hand, Hong et al. (2005) analyzed a
12 000-year proxy record for the East Asian monsoon and concluded that such
abnormal climate conditions could possibly result from frequent and severe El
Niño activities. Using paired oxygen isotope records from North America,
Liu et al. (2014b) indicated that there was a transition from a negative
Pacific North American (PNA)-like pattern during the mid-Holocene to a
positive PNA-like pattern during the late Holocene, which led to drier
conditions in northwestern North America. A similar conclusion was reached by
Finkenbinder et al. (2016) based on lake sediment records from Newfoundland.
They argued that this transition took place around 4.3 ka, leading to wetter
conditions across the Newfoundland region. In contrast, Bond et al. (2001)
argued that North Atlantic SST anomalies around 4.2 ka were related to a
negative North Atlantic Oscillation (NAO) pattern, linked to solar forcing.
Deininger et al. (2017) also found that changes in the atmospheric
circulation associated with northward and southward propagating westerlies
(similar to the NAO but on a millennial instead of a decadal scale) could be
a possible driver of coherency and cyclicity during the last 4.5 kyr, as
seen in multiple speleothem δ18O records that span most of the
European continent. Thus, although there have been many suggested mechanisms,
the ultimate drivers for climatic anomalies at 4.2 ka remain unclear.

Wang (2009a) reviewed studies of Holocene cold events, and concluded that the
most severe Holocene cold event, at ∼8.2 ka, was brought about by
an outburst flood from proglacial Lake Agassiz. This large volume of
freshwater drained into the North Atlantic extremely rapidly, leading to a
brief reorganization of the Atlantic Meridional Overturning Circulation
(AMOC) and a southward displacement of the ITCZ, resulting in dry conditions
over many regions (Barber et al., 1999; Bianchi and McCave, 1999;
Risebrobakken et al., 2003; McManus et al., 2004; Clarke et al., 2004).
Potential external forcing factors for the 4.2 ka BP Event include
non-linear responses to Milankovitch forcing, solar irradiance variations,
and explosive volcanic eruptions, all of which may have brought about
variations in the ocean–atmosphere system (Booth et al., 2005). Wang (2009a)
concluded that solar irradiance minima were the main cause of cold events in
the middle to late Holocene (including the 4.2 ka BP Event) and that internal
oscillations within the climate system could possibly have intensified these
cold events under certain circumstances (Wang, 2009b).

In summary, the 8.2 ka event and corresponding southward shift in the ITCZ
were caused by glacial flooding of the North Atlantic and this can be
reasonably simulated by coupled general circulation models (GCMs) with different boundary
conditions and freshwater forcing (Alley and Agustsdottir, 2005; LeGrande et
al., 2006). By contrast, the forcing mechanisms that brought about the
4.2 ka BP Event are currently uncertain. At 4.2 ka, the major global
monsoon and ocean–atmosphere circulation systems may have been deflected or
weakened synchronously, causing major century-scale precipitation
disruptions, with severe megadroughts over many different regions (Weiss,
2017). As GCM simulations of the 4.2 ka BP Event have not received much
attention, in this study, the spatial patterns and corresponding mechanisms
relevant to the 4.2 ka BP Event are examined and compared to those
associated with the 8.2 ka event.

Data and methodology

Simulations of the last 21 kyr (TRACE-21) were used in this study (He, 2011;
He et al., 2013; Wen et al., 2016). These transient simulations have been
completed using version 3 of the Community Climate System Model (CCSM3),
which is a coupled ocean–atmosphere general circulation model developed by
the National Center for Atmospheric Research (NCAR). The atmosphere model in
the CCSM3 is the Community Atmospheric Model 3 (CAM3) with a horizontal
resolution of ∼3.75∘ (T31), and the ocean model is the Parallel
Ocean Program (POP) with a longitudinal resolution of 3.6∘ and
variable latitudinal resolution.

The “full-forcing” TRACE-21 simulation includes changes in orbital
parameters, greenhouse gases, ice extent (based on the ICE 5G-VM2
configurations), and meltwater fluxes from the Northern Hemisphere and
Antarctic ice sheets. The orbital forcing is based on transient variations of
orbital configuration (Berger, 1978). The concentrations of greenhouse gases
were adopted from Joos and Spahni (2008). The ice sheet data were modified
from the reconstruction of Peltier (2004) and the meltwater scheme was
adopted from Liu et al. (2009).

Simulations in which only one of these factors was included have also been
carried out and are available in the TRACE-21 archive (Otto-Bliesner et al.,
2006; Wen et al., 2016). These simulations can reproduce the timing and
magnitude of many aspects of climate evolution during the last 21 ka, such
as changes in SST (He et al., 2013). However, there
are significant differences between the rate of temperature change in the
model during the early Holocene and many paleoclimatic records (Liu et al.,
2014a; Marcott et al., 2013; Marsicek et al., 2018). In this study, we do not
address this enigma but use the transient model data to compare intervals
within the Holocene when abrupt changes in climate are known to have occurred
in some regions (∼8.2 and ∼4.2 ka). These times were
recently adopted by the International Commission on Stratigraphy as the
chronological boundaries of the early, middle, and late Holocene (Walker et al.,
2012, 2018).

We examine mean annual surface temperature, annual precipitation, and SSTs
from the full-forcing experiment, and also AMOC strength, defined as the
maximum Atlantic stream function between 20 and 50∘ N between 500 and
5000 m depth (Ottera et al., 2010) from the full-forcing and orbital-forcing
experiments.

Results

First, we assess Holocene climate variability as simulated in the
full-forcing experiment. Figure 1 shows the time series of surface
temperature and precipitation over the last 13 kyr. It shows cooling
associated with the Younger Dryas, followed by Holocene warming, but also a
brief cooling episode from ∼8500 to 8000 BP. Thereafter, the record
exhibits strong multi-century-scale variability. Temperature and
precipitation are positively correlated at this global scale. It is tempting
to associate the colder episodes with those identified by Wanner et
al. (2011) or Bond et al. (2001) but only a few of these are coincident in
time.

(a) Northern Hemisphere average surface temperature and
(b) precipitation over the last 13 kyr from the all-forcing
experiment. The blue line is the 10-year running average and the black line
is the 100-year running average. The black dashed line shows the average of
the time series.

The changes of (a) surface temperature (∘C),
(b) precipitation (mm day-1), and (c) SST
(∘C) after 4.5 ka (between 4500–4000 and 4800–4500 BP). The
rectangles in panels (a) and (b) indicate the region with
major dry-farming settlement abandonment around 4.2 ka, according to
Weiss (2016).

The first three patterns (a–c) and principal
components (d–f) of rotated empirical orthogonal function (EOF) modes on the SST over the period
4800–4000 BP.

The period 4.5–4.0 ka was chosen for analysis, by subtracting the mean
annual 2 m air temperatures, SSTs, and precipitation of the period
4500–4000 BP from the preceding period (4800–4500 BP). The spatial
distribution of air temperature (Fig. 2a) shows that temperatures were
significantly colder over most of the extratropical Northern Hemisphere but
generally warmer in the tropics and in the Southern Hemisphere. The main
exceptions are northern South America, which was cooler, and northern India
and Pakistan, which were significantly warmer. Precipitation decreased over
almost all of the Northern Hemisphere, particularly in the tropics where the
ITCZ shifted southward, mainly over South America and adjacent ocean regions,
resulting in higher rainfall in the 0–20∘ S zonal band from 4500 to
4000 BP (Fig. 2b). There was less precipitation over the northern part of
China but more precipitation over southern China, consistent with
paleoclimate reconstructions that indicate a weaker East Asian monsoon (Wang
et al., 2005; Tan et al., 2018a). This pattern is also similar to the
situation during the Little Ice Age (LIA) in China and some of
the megadroughts that have happened in recent centuries (Cook et al., 2010;
Tan et al., 2018b). Over other Asian monsoon regions, such as India, there
were also significant precipitation reductions during the second half of the
5th millennium BP, consistent with speleothem records that show a decline in
Indian summer monsoon rainfall over this period (Kathayat et al., 2017). Over
Central America and the northern edge of South America, conditions were also
drier in the later period, but over the rest of South America, and adjacent
ocean regions, precipitation was higher, due to a southward displacement of
the ITCZ; this pattern is supported by speleothem records of rainfall in
Mexico and Brazil (Lachniet et al., 2013; Bernal et al., 2016). The SST
pattern shows significantly cooler temperatures in the period 4500–4000 BP
over the North Atlantic. This cooling is centered around 50∘ N
(south of Iceland) and extends into the subtropics on the eastern side of the
subtropical gyre. Slightly cooler temperatures are also found over the North
Pacific (Fig. 2c). By contrast, for most of the Southern Hemisphere, there
was a positive change in temperature. Rotated empirical orthogonal function
(EOF) analysis on the global SST field shows the primary feature (in EOFs 1
and 2) to be the cooler SSTs over the North Atlantic, with a shift around
4.5 ka from a predominantly positive to a generally negative pattern
(Fig. 3). This is similar to an AMOC-like pattern over the northern Atlantic
that has been identified in both instrumental and paleoclimatic records
(Delworth and Mann, 2000; Knudsen et al., 2011).

The first three patterns (a–c) and principal
components (d–f) of rotated EOF modes on the SST over the period
9200–8000 BP.

The same evaluation of changes in the 9th millennium BP was made by
subtracting the mean annual 2 m air temperatures, SSTs, and precipitation
from 8800 to 8000 BP from the preceding period (9200–8800 BP), since an
abrupt change in temperature in the model occurred around 8.8 ka
(Fig. 1a). Air temperatures were significantly lower in the second period
over most of the Northern Hemisphere; only a zone from northern South America
across to sub-Saharan Africa and India was warmer in the second period
(Fig. 4a). Almost the entire Southern Hemisphere was warmer. Precipitation
was lower in the second period across all of the Northern Hemisphere,
especially along the ITCZ, which was displaced to the south. This resulted in
increased rainfall in a belt south of the Equator, across almost all of the
tropics (Fig. 4b). The rest of the Southern Hemisphere was also slightly
wetter. SSTs show a strong pattern of cooling over the North Pacific, and the
eastern North Atlantic, south of Iceland, extending around the Atlantic
subtropical gyre into the tropical Atlantic and Caribbean (Fig. 4c). Rotated
EOFs show that the anomalies in the North Atlantic and North Pacific dominate
the first three EOFs (Fig. 5).

The spatial patterns of temperature changes, precipitation changes, and SST
changes were remarkably similar in the late 9th millennium BP and in the period
leading up to the late 5th millennium BP (Fig. 6). The major difference
(Fig. 6a) is that SST changes over the subtropical Atlantic were greater, and
the related changes across the Northern Hemisphere in the 9th millennium BP
were larger than in the late 5th millennium BP. Similarly, the major changes in
precipitation patterns were comparable but less pronounced from
4500–4000 BP. These similarities are somewhat puzzling as the
meltwater forcing sensitivity experiment clearly shows that the 8.2 ka
event was induced by a massive freshwater flux into the Atlantic, whereas
(as far as we know) no comparable meltwater event occurred in the late
Holocene, so it seems unlikely that such forcing was a factor driving the
changes seen in the model output for 4500–4000 BP.

Paleoclimate records have shown that unusually dry conditions persisted for
several centuries around 4.2 ka over many regions, and in some areas
these had devastating societal impacts. In this study, the spatial patterns
of temperature, precipitation, and corresponding circulation anomalies during
the latter part of the 9th and 5th millennia BP (4800–4500 versus
4500–4000 BP and 9200–8800 versus 8800–8000 BP) were
compared based on model simulations. The changes in climate during both
periods were similar and characterized by significant temperature and
precipitation decreases over most of the Northern Hemisphere, whereas the
Southern Hemisphere was slightly warmer and wetter. In particular, the ITCZ
was displaced to the south across much of the globe, and monsoon regions of
the Northern Hemisphere were generally drier. On a regional scale, there was
less precipitation over the northern part of China but more precipitation
over southern China, indicating a reduced eastern Asian summer monsoon.

The 10-year running averaged (blue line) and 100-year running
averaged (black line) time series of AMOC strength, plotted as anomalies from
the mean for 4800–4500 BP from (a) all-forcing experiment
and (b) orbital-forcing experiment. AMOC strength was below the mean
for 63 % of the time in the all-forcing experiment and 87 % of the
time in the orbital-forcing experiment.

It is clear that the earlier period was strongly influenced by freshwater
forcing in the North Atlantic, which drastically reduced the AMOC. The
similarity in anomaly patterns between the 8.2 ka event and the late 5th
millennium BP suggests that there was also disruption to the AMOC in the
later period. However, as there was no comparable freshwater forcing in the
5th millennium BP, we must therefore consider what other factors might have
played a role in reducing AMOC strength. There were no major solar irradiance
changes at that time, so we can rule that out as a forcing factor. However,
there was a major eruption of the Icelandic volcano Hekla at ∼4200 BP,
and it is possible that such an event could have brought about regional
cooling, leading to more extensive, thick sea ice and attendant freshwater
effects on the AMOC (see Moreno-Chamarro et al., 2017). This mechanism deserves further
scrutiny.

The 10-year running averaged (blue line) and 100-year running
averaged (black line) SSTs in the area of the North Atlantic with significant
negative SST differences between the 5th millennium BP and 9th millennium BP
periods (40–60∘ N, 7.5–60∘ W) in Fig. 2c, plotted as
anomalies from the mean for 4800–4500 BP; ∼69 % of the time,
temperatures in this region were below the mean.

In the all-forcing TRACE-21 simulation, AMOC strength declined slightly
during the late Holocene and underwent multi-century fluctuations (Fig. 7a),
which were strongly correlated with SSTs in the region of the North Atlantic
where cooling was so prominent from 4.5 to 4.0 ka (Fig. 8). Mean SSTs in
this region over the last 4500 years of the model simulation stayed below the
4.8–4.5 ka average for ∼69 % of the time (Fig. 8), and AMOC
strength was similarly below the 4.8–4.5 ka mean for 63 % of the
time (Fig. 7a). One of these fluctuations was associated with an AMOC minima
around 4.2 ka. In the TRACE-21 model simulation with only orbital
forcing, AMOC strength reached its Holocene maximum around 4.8 ka, then
slightly weakened (by ∼10 %) over the late Holocene, staying below
the 4.8–4.5 ka mean for 87 % of the time, with minor multi-century
variations superimposed on the long-term downward trend (Fig. 7b). This
suggests that a combination of factors, led by long-term changes in
insolation, drove a steady decline in SSTs across the North Atlantic and a
reduction in the AMOC, with associated teleconnections across the globe
(including drought in some regions). Minor fluctuations around this declining
trend were the dominant pattern for most of the last 4500 years. This
interpretation helps explain widespread paleoclimatic evidence for the onset
of neoglaciation around 5000–4500 BP, followed by a series of neoglacial
advances and retreats during recent millennia (Porter, 2000; Barclay et al.,
2009; Solomina et al., 2015; Bradley and Bakke, 2019). Since the onset of
neoglaciation early in the 5th millennium BP, mountain glaciers fluctuated
in extent but did not entirely disappear, indicating that a distinctly
different climate state prevailed compared to the period prior to ∼5 ka, when many mountain regions were ice-free.

We therefore conclude from the model simulations that the 4.2 ka BP Event
was one of several late Holocene multi-century fluctuations that were
embedded in a long-term, low-frequency change in climate that occurred after
∼4.8 ka. Worldwide climatic anomalies during these fluctuations were
driven by changes in the strength of the AMOC and related teleconnections.
Whether such multi-century fluctuations were a response to internal
centennial-scale ocean–atmosphere variability (see Yan et al., 2018), external forcing (such as explosive
volcanic eruptions and associated feedbacks), or a combination of such
conditions is not known. Further studies of the role of both external forcing
and internal variability are needed to provide a better understanding of such
mechanisms (see Ottera et al., 2010;
Moreno-Chamarro et al., 2017; Gupta and Marshall, 2018).

The TraCE-21ka data used in this study can be accessed
through the website of Earth System Grid
(https://www.earthsystemgrid.org/project/trace.html, last access:
1 June 2018).

LN, RSB, and MY designed the experiments and carried them
out. LN and MY performed the formal analyses. LN, RSB, and MY performed the
investigation. LN and JL contributed to funding acquisition. JL and RSB
provided supervision. LN and RSB prepared the paper with contributions
from all co-authors.

The authors declare that they have no conflict of
interest.

This article is part of the special issue “The 4.2 ka BP
climatic event”. It is a result of “The 4.2 ka BP Event: An International
Workshop”, Pisa, Italy, 10–12 January 2018.

Acknowledgements

This research was jointly supported by the National Key Research and
Development Program of China (grant no. 2016YFA0600401), the National Natural
Science Foundation of China (grant no. 41501210, grant no. 41420104002, grant
no. 41671197, and grant no. 41631175), the Jiangsu Province Natural Science
Foundation (grant no. BK20150977), Top-notch Academic Programs Project of
Jiangsu Higher Education Institutions (grant no.
PPZY2015B115), the Program of Innovative Research Team of Jiangsu Higher Education Institutions of China, and the Priority
Academic Development Program of Jiangsu Higher Education Institutions (grant
no. 164320H116). Support was also received from US NSF grant PLR-1417667 to
the University of Massachusetts. TraCE-21ka was made possible by the DOE
INCITE computing program and supported by NCAR, the NSF P2C2 program, and the
DOE Abrupt Change and EaSM programs. Edited
by: Harvey Weiss Reviewed by: two anonymous referees